1. Field of the Invention
The present invention relates to techniques for evaluating a subsurface formation using a probe assembly conveyed on a downhole tool positioned in a wellbore penetrating the subsurface formation. More particularly, the present invention relates to techniques for reducing the contamination of formation fluids drawn into and/or evaluated by the downhole tool via the probe assembly.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A string of downhole pipes and tools with a drill bit at an end thereof, commonly known in the art as a drill string, is advanced into the ground to form a wellbore penetrating (or targeted to penetrate) a subsurface formation of interest. As the drill string is advanced, a drilling mud is pumped down through the drill string and out the drill bit to cool the drill bit and carry away cuttings and to control downhole pressure. The drilling mud exiting the drill bit flows back up to the surface via the annulus formed between the drill string and the wellbore wall, and is filtered in a surface pit for recirculation through the drill string. The drilling mud is also used to form a mudcake to line the wellbore.
It is often desirable to perform various evaluations of the formations penetrated by the wellbore during drilling operations, such as during periods when actual drilling has temporarily stopped. In some cases, the drill string may be provided with one or more drilling tools to test and/or sample the surrounding formation. In other cases, the drill string may be removed from the wellbore (called a “trip”) and a wireline tool may be deployed into the wellbore to test and/or sample the formation. Such drilling tools and wireline tools, as well as other wellbore tools conveyed on coiled tubing, are also referred to herein simply as “downhole tools.” The samples or tests performed by such downhole tools may be used, for example, to locate valuable hydrocarbons and manage the production thereof.
Formation evaluation often requires that fluid from the formation be drawn into a downhole tool for testing and/or sampling. Various devices, such as probes and/or packers, are extended from the downhole tool to isolate a region of the wellbore wall, and thereby establish fluid communication with the formation surrounding the wellbore. Fluid may then be drawn into the downhole tool using the probe and/or packer.
A typical probe employs a body that is extendable from the downhole tool and carries a packer at an outer end thereof for positioning against a sidewall of the wellbore. Such packers are typically configured with one relatively large element that can be deformed easily to contact the uneven wellbore wall (in the case of open hole evaluation), yet retain strength and sufficient integrity to withstand the anticipated differential pressures. These packers may be set in open holes or cased holes. They may be run into the wellbore on various downhole tools.
Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings are radially expanded about a downhole tool to isolate a portion of the wellbore wall therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the downhole tool via the isolated portion of the wellbore.
The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making the appropriate seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet therein by lowering the pressure in the downhole tool. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. Patent Application No. 2004/0000433.
Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the downhole tool. However, it has been discovered that when the formation fluid passes into the downhole tool, various contaminants, such as wellbore fluids and/or drilling mud may, and often do, enter the tool with the formation fluids. The problem is illustrated in FIG. 1, which depicts a subsurface formation 16 penetrated by a wellbore 14 and containing a virgin fluid 22. A layer of mud cake 15 lines a sidewall 17 of the wellbore 14. Due to invasion of mud filtrate into the formation during drilling, the wellbore is surrounded by a cylindrical layer known as the invaded zone 19 containing contaminated fluid 20 that may or may not be mixed with the desirable virgin fluid 22 that lies in the formation beyond the sidewall of the wellbore and surrounds the contaminated fluid 20. Since the contaminates 20 tend to be located near the wellbore wall 17 in the invaded zone 19, they may affect the quality of measurements and/or samples of the formation fluids. Moreover, contamination may cause costly delays in the wellbore operations by requiring additional time for more testing and/or sampling. Additionally, such problems may yield false results that are erroneous and/or unusable.
FIG. 2A shows the typical flow patterns of formation fluids as they pass from a subsurface formation 16 into a wireline-conveyed downhole tool 1a. The downhole tool 1a is positioned adjacent the formation 16 and a probe 2a is extended from the downhole tool through the mudcake 15 to sealingly engage the sidewall 17 of the wellbore 14. The probe 2a is thereby placed in fluid communication with the formation 16 so that formation fluid may be passed into the downhole tool 1a. Initially, as shown in FIG. 1, the invaded zone 19 surrounds the sidewall 17 and contains contaminates 20. As a pressure differential is created by the downhole tool 1a to draw fluid from the formation 16, the contaminated fluid 20 from the invaded zone 19 is first drawn (not particularly shown in FIG. 1 or 2A) into the probe thereby producing fluid unsuitable for sampling. However, after a certain amount of contaminated fluid 20 passes through the probe 2a, the virgin fluid 22 breaks through the invaded zone 19 and begins entering the downhole tool 1a via the probe 2a. More particularly, as shown in FIG. 2A, a central portion of the contaminated fluid 20 flowing from the invasion zone 19 into the probe gives way to the virgin fluid 22, while the remaining portion of the produced fluid is contaminated fluid 20. The challenge remains in adapting to the flow of the formation fluids so that the virgin fluid is reliably collected in the downhole tool 1 a during sampling.
FIG. 2B shows the typical flow patterns of formation fluids as they pass from a subsurface formation 16 into a drill string-conveyed downhole tool 1b. The downhole tool 1b is conveyed among one or more (or itself may be) measurement-while-drilling (MWD),logging-while-drilling (LWD), or other drilling tools that are know to those skilled in the art. The downhole tool 1b may be disposed between a tool or work string 28 and a drill bit 30, but may also be disposed in other manners know to those or ordinary skill in the art. The downhole tool 1b employs a probe 2b to sealingly engage and draw fluid from the formation 16, in similar fashion to the downhole tool 1a and probe 2a described above.
It is therefore desirable that sufficiently “clean” or “virgin” fluid be extracted or separated from the contaminated fluid for valid testing. In other words, the sampled formation fluid should have little or no contamination. Attempts have been made to eliminate contaminates from entering the downhole tool with the formation fluid. For example, as depicted in U.S. Pat. No. 4,951,749, filters have been positioned in probes to block contaminates from entering the downhole tool with the formation fluid.
Other techniques directed towards eliminating contaminates during sampling are provided by published U.S. Patent Application No. 2004/0000433 to Hill et al. and U.S. Pat. No. 6,301,959 to Hrametz et al., the entire contents of both being hereby incorporated by reference. FIGS. 3 and 4 are schematic illustrations of the probe solution disclosed by the Hrametz patent. Hrametz describes a fluid sampling pad 13 mechanically pressed against the borehole wall. A probe tube 18 extends from the center of the pad and is connected by a flowline 23a to a sample chamber 27a. A guard ring 12 surrounds the probe and has openings connected to its own flowline 23b and sample chamber 27b. This configuration is intended to create zones so that fluid flowing into the probe is substantially free of contaminating borehole fluid.
Despite such advances in fluid sampling, there remains a need to reduce contamination during formation evaluation. In some cases, cross-flow between adjacent flowlines may cause contamination therebetween. It is desirable that techniques be provided to assist in reducing the flow of contamination of formation fluid entering the downhole tool and/or isolate clean formation fluid from contaminates as the clean fluid enters the downhole tool. It is further desirable that such a system be capable of one of more of the following, among others: providing a good seal with the formation; enhancing the flow of clean fluid into the tool; optimizing the flow of fluid into the downhole tool; avoiding contamination of clean fluid as it enters the downhole tool; separating contaminated fluid from clean fluid; optimizing the flow of fluid into the downhole tool to reduce the contamination of clean fluid flowing into the downhole tool; and/or providing flexibility in handling fluids flowing into the downhole tool.